They already have the N20/6MT combo standardized for the US and the N55/6MT plus N55/6MT/xDrive also have been standardized in the F30. Why not just bring it to the higher 35i, or just bring it to the base sDrive 28i? It will barely cost them anything

Unfortunately, to pass US emissions and safety standards, they need to test each drivetrain combination separately. So, a manual, AWD, 2.0T X1 is tested separately than an automatic, AWD, 2.0T, X1. Each combination of model, engine, and transmission is expensive to certify.